To switch language, log in to Trello, then go to the user profile menu in the top right corner of the page. Click on 'Change Language...' to choose your preferred language.

"Change Language" will show up in the current language, so if you're not sure which option to choose, it's the next-to-last item on the menu:

Switching language for our Trello mobile apps isn't currently an option. Trello will automatically use the language that you've selected as the system language for your device.

If you'd like to help with translating Trello into other languages, take a look at https://trello.com/translations to see what other languages we're working on and find out more on how to become a translator.